1. Field of the Invention
The present invention relates to a method of making a mask pattern and a mask used in photolithography techniques for ultra large scale integration (ULSI) and particularly to a method of making a mask pattern including a microcircuit pattern (an actual pattern) which will be called target pattern in the following description and an alignment mark pattern for overlaying and a mask formed through the method.
2. Description of the Related Art
As the demand for achieving microstructural semiconductor chips grows, target pattern dimensions have been reduced down to a width of 0.2 to 0.35 .mu.m. The demand for overlay accuracy increases with the reduction in target pattern width. That is, a ULSI circuit has different structures in depth and requires masks with microcircuit patterns different among depths. If overlay accuracies among the masks and a wafer are low, manufacturing yields are reduced. In order to improve overlay accuracy, methods are generally known for improving stage accuracy, measurement systems and manufacturing processes, if categorized broadly. In photolithography techniques in particular, improvements in formation of an alignment mark for overlaying a mask on a wafer are required.
More than a decade of research and development has been made on microstructure techniques of mask patterns (target patterns) with photolithography techniques and has demonstrated great improvements. The improvements greatly owe to a short-wavelength exposure light source, a high numerical aperture (NA) of lens and improvements in resists. For forming such fine target patterns, an exposure system is used such as a reduction-type projection printing stepper using i-lines of mercury lamp (wavelength of 365 nm) and excimer laser of KrF, for example (wavelength of 248 nm). While improvements have been made in microstructure techniques of target patterns, no great modifications have been made on alignment mark geometries since a stepper was first introduced.
However, there are problems in keeping on using alignment marks previously developed. This is partly due to a change in resists to a chemically amplified resist used for excimer laser such as KrF of short wavelength. Solubility of the chemically amplified resist is improved with the aid of reaction produced by an exposure. In forming microscale patterns, the chemically amplified resist offers excellent line width linearity or critical dimension (CD) linearity. In forming a greater pattern (10 to 12 .mu.m) such as an alignment mark, however, it is known that a great deviation from a designed size may occur due to reduced linearity. Such deviation in alignment mark size affects overlay accuracy (alignment accuracy) of a target pattern for printing a fine target pattern and particularly affects scaling for writing an enlarged or reduced pattern of original. In addition to such a problem due to reduced linearity, degradation in mark geometries is known due to the use of high numerical aperture lens and a halftone phase shift mask (HT-PSM) for achieving finer patterns. This is because an exposure is often made regardless of resist linearity (such as an over exposure with greater exposure energy) in order to form a target pattern of size around the resolution limit (248 nm) of the resist or below.
Deviation in alignment marks resulting from the factors as described so far is often seen. FIG. 1A to FIG. 1D illustrates examples of degradation of alignment mark patterns of LSA marks of 5 to 10 .mu.m, for example. FIG. 1A illustrates a design pattern of an alignment mark pattern 200 for overlaying a micro target pattern 100 of 0.2 to 0.3 .mu.m in line width. FIG. 1B illustrates a plane geometry of printed pattern of alignment mark pattern 200 with a typical resist (novolak base resist) for i-lines and g-lines used in a process for forming a relatively rough pattern. FIG. 1C illustrates a plane geometry of printed pattern of alignment mark pattern 200 with a chemically amplified resist for a KrF excimer laser used in a process for forming a relatively fine pattern and with an over exposure. FIG. 1D illustrates a plane geometry of printed pattern of alignment mark pattern 200 formed further with an HT-PSM as a mask in a condition similar to that of FIG. 1C. With the typical novolak base resist, as shown in FIG. 1B, an optical proximity effect is only produced in corners of the mark. As shown in FIG. 1C, the overall mark geometry is reduced with the chemically amplified resist and with an over exposure. The pattern is further reduced with the HT-PSM, as shown in FIG. 1D. The tendency of reduction depends on a resist, line width of a target and so on. Enlarging and reduction of a design pattern are thus repeated.
As described so far, an alignment mark pattern is degraded and great deviation from a design pattern results, due to the use of resists different among pattern formation processes, the use of an HT-PSM and so on. Such deviation of alignment mark patterns greatly affects overlay accuracy of micro target patterns. Product yields will be therefore reduced.